1. Technical Field
The present invention relates to a fixing device, and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, which employs such a fixing device.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (a “fuser member”) and the other being pressed against the heated belt or roller (a “pressure member”), which together form a heated area of contact called a fixing nip, through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
For example, one such fixing device includes a rotatable, elastic fuser belt paired with an elastically biased, rotatably driven pressure roller. The fuser belt is looped into a generally cylindrical configuration for rotation around a thermally conductive heat pipe, within which a heater is situated to radiate heat to the heat pipe from inside the loop of the fuser belt. The pressure roller is disposed opposite a fuser pad, which is disposed inside the loop of the fuser belt at the area of the fixing nip to support the fuser belt against pressure from the pressure roller, with the fuser belt interposed between the fuser pad and the pressure roller. The fixing device also includes a biasing mechanism, such as a spring, which elastically biases the pressure roller against the fuser pad through the fuser belt to form the above-described fixing nip therebetween.
Upon activation, the fixing device initially heats the fuser belt to an operational temperature during warm-up, in which the heater heats the heat pipe to conduct heat to the fuser belt, whereas the pressure roller rotates to in turn rotate the fuser belt to allow uniform heating in the circumferential, rotational direction of the fuser belt. After completion of warm-up, a recording medium is conveyed through the fixing nip, at which a toner image on the incoming medium is fixed in place with heat from the fuser belt and pressure from the pressure roller pressing against the fuser belt.
The heat pipe employed in this type of fixing device is configured as a longitudinally slotted metal tube formed by bending a sheet of metal, such as stainless steel, into a generally cylindrical configuration, with a pair of opposed longitudinal edges thereof folded inward and spaced apart from each other to define a concave, longitudinal slot therebetween that extends the entire length of the heat pipe. The heat pipe is inserted into the loop of the fuser belt while accommodating the fuser pad in its longitudinal side slot, so that an outer circumferential surface of the heat pipe adjoins an inner circumferential surface of the fuser belt except at the fixing nip where the fuser pad is disposed in the assembled fixing device.
One problem encountered when using such a longitudinally slotted heat pipe in the fixing device is deformation of the heat pipe, in which the pair of opposed longitudinal edges of the metal sheet gradually separates further from each other to enlarge the gap or opening therebetween due to an elastic recovery of the bent sheet of metal, known in the art as “springback”. If not corrected, such deformation of the heat pipe would result in localized sliding contact between the heat pipe and the fuser belt that accelerates wear and tear of the fuser belt.
To address this problem, one possible approach is to provide a fastener or connecting member to fix the opposed, longitudinal slot-defining edges of the heat pipe in position with respect to each other. For example, such a fastener may be a pair of mechanical clamps or stays shaped with a rectangular U-shaped cross section to conform to the folded configuration of the opposed longitudinal edges of the heat pipe. These stays are fitted to the longitudinal side slot, one from inside and the other from outside the heat pipe, so as to clamp together the opposed longitudinal edges, thereby retaining the heat pipe in the desired shape. However, the stays are unsatisfactory in that they complicate assembly and increase costs.